High efficiency near-field electromagnetic probe having a bowtie antenna structure

ABSTRACT

A near field electromagnetic probe converts an incident energy beam into an interrogating beam which exhibits, in the near field vicinity of the probe, a transverse dimension that is small in relation to the wavelength of the incident energy beam. The probe comprises an energy source for providing the incident energy beam with a wavelength λ. An antenna is positioned in the path of the incident energy beam and comprises at least a first conductive region and a second conductive region, both of which have output ends that are electrically separated by a gap whose lateral dimension is substantially less than λ. The electromagnetic system which produces the incident energy should preferably have its numerical aperture matched to the far-field beam pattern of the antenna. Further, the incident beam should have a direction of polarization which matches the preferred polarization of the antenna. The near field probe system of the invention can also sense fields in the near field gap and reradiate these to a far-field optical detector. Thus the probe can serve to both illuminate a sample in the near field gap, and to collect optical signals from an illuminated sample in the near field gap.

FIELD OF INVENTION

This invention relates to near field microscopy and, more particularly,to an improved near field electromagnetic probe which exhibitsextraordinary levels of transmission efficiency.

BACKGROUND OF THE INVENTION

In the field of optical microscopy, it is generally acknowledged thatthe limit of resolution is approximately one-half of the wavelength ofthe illuminating light or approximately 300 nanometers for visiblelight. However, as devices and features to be imaged push further intothe nanometric regime, the limits of resolution of optical microscopybecome obstacles to be overcome so as to achieve visualization of theever smaller feature sizes. Investigators have used shorter wavelengthradiation such as electron or X-ray microscopy to image in the submicronregion. Others have employed various forms of scanning probe microscopyin such imaging, wherein the scanning tunneling microscope is awell-known example.

Recently, interest has grown in the field of near field optics as amethod for overcoming the resolution limits of optical microscopy. Forinstance, see "Near Field Optics: Microscopy, Spectroscopy, and SurfaceModification Beyond the Diffraction Limit", Betzig et al., Science, Vol.257, 10 Jul. 1992, pages 189-195.

Betzig et al. indicate that Fourier optics demonstrate that thediffraction limit to resolution in optical microscopy is not afundamental limitation, but rather arises from the assumption that thedetection element (e.g. a lens) is typically many wavelengths away fromthe sample being imaged. By laterally scanning a source or a detector oflight in close proximity to the sample, an image can be generated whichexhibits a resolution that is functionally dependent on only the lightsource (i.e., probe) size and the probe-to-sample separation. Each ofthese dimensions can, in principal, be made much smaller than thewavelength of light.

Betzig et al. further indicate that the prior art first suggested that anear field optical probe be constructed by use of asubwavelength-diameter aperture in an optically opaque screen. Lightincident upon one side of the screen would be transmitted through theaperture and, if the sample was within the near field, would illuminateonly one small region at any one time. This technique led to images, inthe centimeter wavelength range, at 1/60 of the wavelength of theilluminating energy.

To obtain a narrow probe to follow surface contours, Lewis et al. (U.S.Pat. No. 4,917,462) constructed a near field optical probe from a drawnpipette. The outside was metallized, with an aperture at the end. Pohl(U.S. Pat. No. 4,604,520) constructed a near field optical probe, usinga metallized, pointed quartz rod with an opening in the metal at thepointed end. Both approaches suffered significant intensity losses andscattering of the light before reaching the aperture at the end of theprobe.

To achieve improved reliability of fabrication and greater probe beamintensities, Betzig et al. (U.S. Pat. No. 5,272,330) developed a nearfield optical probe that comprises a tapered optical fiber coated withan aluminum layer. Laser light was coupled into the optical fiber and,after transmission through the fiber, the laser light exited at anaperture at the apex of the probe and illuminated a sample positioned inthe near field. The illuminating light, after interaction with thesample, was collected by a conventional lens system. The tapered fiberprobe yielded a resolution of down to 12 nanometers, or less than 1/40of the wavelength of visible light.

Notwithstanding the ability to achieve levels of resolutionsubstantially less than the wavelength of the illuminating energy, allknown near field optical probes exhibit low levels of transmissionefficiency. This is not surprising as the essential feature of prior artoptical probes is to condense a relatively large cross sectionelectromagnetic beam and to pass it through a very tiny aperture, theresult being substantial energy transmission losses in the process. Thetapered fiber probe of Betzig et al. exhibits improved transmissionefficiencies over other prior art near field probes, however, itsefficiency is still orders of magnitude smaller than unity. For example,a probe diameter of 1/20 of a wavelength will have a transmissionefficiency of the order of 10⁻⁵ to 10⁻⁶. This low efficiency limits theuse of the tapered fiber probe to applications with either a largephoton flux or a slow acquisition rate of reflected or transmittedenergy. A large photon flux is often undesirable for biological ormolecular systems which are subject to damage or bleaching whensubjected to such a flux. Also, applications in which light is sentthrough and also collected via a tapered fiber suffer large intensitylosses during each transit through the aperture.

Another group has attempted to improve the efficiency of a near fieldprobe through the use of a transmission line, such a coaxial line orstrip line, to collect and feed the incident energy to the limitingscreen aperture. See: Fee et al., "Scanning Electromagnetic TransmissionLine Microscope with Sub-Wavelength Resolution", Optics Communications,Vol. 69, No. 3.4, January 1989, pages 219-224. Fee et al. describe amicroscope wherein an electromagnetic wave is funneled into anopen-ended coaxial line. The center conductor of the coaxial line isconfigured as a scanning tip and is passed through a groundplane/aperture so as to create an emitting near-field probe.

Fee et al. describe an ultraminiaturized microstrip patch antenna andconnected coaxial transmission line wherein a small metal patchapproximately 1/2 wavelength in diameter is deposited on a transparentsubstrate. The patch forms the body of an antenna, to be illuminatedthrough the transparent substrate. Next, a transparent dielectric layerwith a thickness of some fraction of a wavelength is deposited on top ofthe patch and finally, an opaque metal film is applied over thetransparent dielectric and forms both a ground plane for the antenna andthe outer conductor of the coaxial transmission line. An aperture isopened in the ground plane, exposing the antenna patch. A scanning tipis then deposited in contact with the metal patch.

Thus, the final configuration comprises an antenna which is illuminatedthrough the substrate to cause an induced field to be created betweenthe patch antenna and the ground plane. The resultant field is thenchanneled by the coaxial structure to the emitting probe which extendsthrough a small aperture in the ground plane.

Fee et al. indicate that the electrical energy density inside thewaveguide structure is substantially improved over that achieved by asimple circular aperture. They further indicate that the fieldenhancement achieved by the transmission line will provide much highersensitivities, but will limit the amount of power that can be handled,without melting or dielectric breakdown.

Fee et al., along with all known investigators in the near fieldmicroscopy area, have continued to employ a construct which essentiallycompresses the incident energy so that it can be transmitted, at leastin part, through an aperture in an opaque film. As a result, substantialenergy losses result.

Accordingly, it is an object of this invention to provide an improvednear field electromagnetic probe.

It is another object of this invention to provide an improved near fieldelectromagnetic probe which exhibits a high level of efficiency, asbetween incident and transmitted energy.

SUMMARY OF THE INVENTION

A near field electromagnetic probe converts an incident energy beam intoan interrogating beam which exhibits, in the near field vicinity of theprobe, a transverse dimension that is small in relation to thewavelength of the incident energy beam. The probe comprises an energysource for providing the incident energy beam with a wavelength λ. Anantenna is positioned in the path of the incident energy beam andcomprises at least a first conductive region and a second conductiveregion, both of which have output ends that are electrically separatedby a gap whose lateral dimension is substantially less than λ. Theelectromagnetic system which produces the incident energy shouldpreferably have its numerical aperture matched to the far-field beampattern of the antenna. Further, the incident beam should preferablyhave a direction of polarization which matches the preferredpolarization of the antenna.

The conductive regions respond to the incident energy beam byestablishing a field across the gap which separates the output ends,which field causes re-radiation of an interrogating beam. A target ispositioned in sufficient proximity to the gap to be within the nearfield distance thereof.

The near field probe system of the invention can also sense fields inthe near field gap and reradiate these to a far-field optical detector.Thus the probe can serve to both illuminate a sample in the near fieldgap, and to collect optical signals from an illuminated sample in thenear field gap. Because of high transmission efficiencies, the probe canbe used as part of a reflection optical microscope, wherein illuminationand collection of optical energy are both performed via antennare-radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a near-field electromagnetic probeincorporating the invention.

FIG. 2 is a sectional view of the near-field electromagnetic probe ofFIG. 1, along line 2--2.

FIG. 3 is a schematic view of an imaging system incorporating anelectromagnetic probe incorporating the invention.

FIG. 4a is a plot of field intensity profile for an electromagnetic beamthat is incident upon the antenna of FIG. 1, the incident beam having afrequency of 2.15 GHz.

FIG. 4b is a plot of field intensity profile derived 2 mm beyond theplane of the antenna.

FIG. 5 is a plot of detected power along the direction of propagation,as measured in the far-field, with (i) a feed waveguide only present and(ii) with both the feed waveguide and the antenna present.

FIG. 6 is a plot of calculated surface wave decay length as a functionof photon wavelength for both aluminum and gold antenna structures.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1-3, a near-field electromagnetic probe 10(hereafter referred to as "probe"), comprises an electromagneticallytransparent substrate 12 upon which an antenna structure 14 issupported. In the form shown in FIGS. 1 and 2, antenna structure 14comprises a bowtie antenna including conductive arms 16 and 18,respectively. At terminations 20 and 22, conductive arms 16 and 18 areseparated by a gap 24 having a transverse dimension d. In essence,conductive arms 16 and 18 form a dipole-like antenna. Other antennastructures will work with the invention, such as log-periodic, spiraland slot antennas.

Gap 24 forms an emission "region" between terminations 20 and 22 ofconductive arms 16 and 18. The transverse dimension "d" betweenterminations 20 and 22 is small in relation to the wavelength of theincident electromagnetic energy.

It is preferred that the incident energy have a wavelength in theoptical range, however, it is to be understood that the invention isequally applicable to non-optical wavelength applications. Incidentenergy beam spot 26 is indicated by a circular dashed line in FIG. 1 andpreferably exhibits a diffraction limited spot diameter, i.e., not morethan 2λ.

From a review of FIGS. 1 and 2, it can be seen that terminations 20 and22, separated by gap 24, constitute a capacitance. In order to moreefficiently impedance match the capacitance of gap 24 to the antennastructure, and improve the coupling of energy thereunto, it is preferredto connect an inductor 28 in parallel with region 24 to create a tunedcircuit. The essential idea is to match the antenna impedance to theradiation resistance of the dipole radiator formed at gap 24.

As shown in FIGS. 2 and 3, substrate 12 is positioned sufficiently closeto a target 30 (positioned on a support 32) to place the upper surfaceof target 30 in the near-field of probe energy created by the fieldcreated within gap 24. Antenna structure 14 is illuminated (viahalf-silvered mirror 34) by a polarized beam 36, whose direction ofpolarization 38 is oriented along the direction of preferredpolarization of antenna structure 14. An optic 40 is positioned abovehalf-silvered mirror 34 and focuses the reradiated energy from antennastructure 14 onto a detector 41.

The underlying and critical finding in this invention is that incidentbeam spot 26 is acted upon by conductive arms 16 and 18 in such a manneras to localize substantially all of the energy within incident beam spot26 in the vicinity of gap 24 between terminations 20 and 22. Thus, eventhough substrate 12 is transparent at the wavelength of incident beamspot 26, antenna structure 14 essentially "collapses" the incident beamspot by concentrating its energy into currents on conductive arms 16 and18 and focusing the induced currents to output terminals 20 and 22.Since output terminals 20 and 22 are nearly open-circuited at thefrequency of incident beam spot 26, charge accumulates at terminations20 and 22, resulting in a displacement current flow across gap 24. Thedisplacement current causes a re-radiation similar to that of a Hertziandipole of dimension d.

Since gap 24 can be made much smaller than the wavelength of theincident energy spot 26, a beam 42 is created (see FIG. 2) which, innear-field 43, has a cross-sectional diameter approximately equal to d.Thus, in near-field 43, interrogating beam 42 is created, having thefrequency of the incident beam, but a transverse dimension that is smallin relation to the wavelength of the incident beam. Importantly, nopinhole or other apertured plate is required to achieve interrogatingbeam 42.

In the collection mode, aperture 24 is placed close to sample 30.Aperture 24 senses electromagnetic emissions from target 30 in nearfield region 43 (as a result of off-axis illumination, illumination frombeneath target 30 or self induced emanations, e.g., fluorescence).Currents are induced thereby into antenna arms 16 and 18 and, as aresult, the induced energy is reradiated up through optic 45, mirror 34and is focussed by optic 40 onto detector 41. Probe 10 serves as areflection microscope when an interrogating beam 42 is reflected fromsample 30 back to gap 24. In the same manner as for the collection mode,currents are induced into antenna arms 16 and 18 and the induced energyis reradiated via optic 45, mirror 34 to optic 40.

Experimental Results

To demonstrate that the antenna of FIGS. 1 and 2 generates localizedradiation, a scale model system was built at 2.15 GHz (λ=14cm). Theexperimental configuration was similar to that shown in FIGS. 1 and 2;however, incident beam 26 was formed by an open-ended rectangularwaveguide positioned 2.5 cm in the front of bowtie antenna 14. Theantenna had an opening angle 50 of 90° and a total length of 30 cm. Theoutput terminals of the antenna formed a square gap of width 7 mm orapproximately (1/20)λ. The field intensity produced by the antenna wasdetected using a dipole probe of dimension λ/20, integrated with acoaxial balun. The dipole probe measured local energy density or fieldintensity.

As shown in FIG. 4a, the field intensity profile due to the waveguide(i.e. without antenna 14 positioned in front of the waveguide) isroughly Gaussian with a diameter on the order of 7 cm or approximatelyλ/2.

The field intensity radiated by the bowtie antenna was measured byscanning a dipole probe, positioned a fixed distance from the plane ofthe bowtie on the side of the antenna structure opposite to that fromwhich it was irradiated. An image taken 2 mm (λ/75) in the near fieldemission area of the bowtie, is shown in FIG. 4b. The field was stronglylocalized in gap 24 of the antenna and evidenced a transverse dimensionof approximately λ/20.

The unique aspect of the invention is that it exhibits very largetransmission efficiencies. Shown in FIG. 5 are measurements of intensitytaken along the Z axis (perpendicular to the plane of antenna 14). Plot50 shows the intensity as measured without antenna 14 present (i.e., theopen waveguide). Plot 52 shows the measured field intensity (x10) withantenna 14 present. Both curves scale as an inverse square law in thefar-field. The ratio of the far-field intensities suggests atransmission efficiency of the order of 5%. While this efficiency is, atbest, an estimate (because it does not properly take into accountdifferences between the far-field patterns of the antenna and the openwave guide), it is clear that the transmission efficiency of probe 10 ismany orders of magnitude improved over the prior art tapered fiberprobes.

To achieve a near unity transmission efficiency, the impedance of theantenna structure should be matched to the impedance of the capacitancecreated between output terminals 20 and 22. For instance, bowtie antenna14 (without tuning inductor 28) could be shown to have an impedance ofapproximately 188 ohms. The antenna is coupled to gap 24 which lookslike a capacitor in series with the radiative resistance of a Hertziandipole. The radiative resistance of a Hertzian dipole of length d=λ(approximately 7 mm) can be shown to be approximately 2 ohms. Providedthe capacitance is large enough, the efficiency calculated is in theorder of 4%. This compares well with the experimental results andindicates that proper antenna design will yield higher efficiencies.

While the above probe structure has been described in the context of abowtie structure, those skilled in the art will realize that otherantenna structures which provide a high field strength acrosselectrically isolated terminals are equally useful in generatingnear-field radiative energy beams. Further, while the invention wastested utilizing a 2.15 GHz incident beam, a preferred embodiment willutilize an optical beam in the visible region of the electromagneticspectrum as the incident beam, thereby creating an induced interrogatingbeam in the visible.

The invention can be incorporated into a scanned probe microscope viamany techniques. For instance, the antenna can be fabricated on thebottom of a solid immersion lens or designed as a part of conventionalatomic force probe. The highest energy photons for which the antenna canbe used will be limited by the finite absorption of light in the metalthat defines the antenna. The effect of the finite conductivity can bedetermined quantitatively by considering the propagation of surfacewaves at a metal-air interface. The decay length of a wave along thesurface of a metal antenna can be calculated and is plotted in FIG. 6for aluminum and gold across the visible spectrum, using values reportedin the literature. Since the antenna will be illuminated by adiffraction limited laser spot, the surface wave needs to propagate, atmost, of the order of λ.

From the plot of FIG. 6, it is clear that gold is the metal of choicefor wavelengths as short as 0.7 microns. Using an antenna arm length of10λ as a conservative criteria, aluminum will work efficiently to thenear ultraviolet.

Use of inductor 28 was shown to dramatically increase the transmissionefficiency as measured in the microwave frequency region. Inductor 28forms a resonant circuit with the gap capacitance, and thisinductor-capacitor combination forms a resonant circuit at the frequencyof the incident microwave beam. For optical frequencies, variousapproaches can be used to implement an element which acts as aninductor. Metal structures and also various dielectric materials withresonances at frequencies near that of the incident beam, may be used.Alternatively, a resonant optical cavity may be utilized in the inputbeam so that there is more efficient coupling of the incident beamenergy to the radiation resistance of the gap. This may be implementedin an integrated form if the antenna is mounted on the end of an opticalfiber, so that a fiber resonator can then be used.

Reflections of incident energy may occur from the antenna structure inthe absence of a target and be detected by detector 41. To distinguishsuch reflections from energy reflected from a target, the target isscanned in the two planar dimensions and the reflection data from theantenna structure is subtracted as a background signal. This techniqueis needed when the observing wavelength is the same as the incidentwavelength. If instead, the target emanates a different wavelength(i.e., fluoresces), a filter can be used to exclude the reflections dueto the incident beam.

It should be understood that the foregoing description is onlyillustrative of the invention. Various alternatives and modificationscan be devised by those skilled in the art without departing from theinvention. For instance, while the probe described above has beendescribed in both illumination and collection configurations, it can beused simultaneously for both functions if the illumination andcollection wavelengths are offset or are differently polarized.Accordingly, the present invention is intended to embrace all suchalternatives, modifications and variances which fall within the scope ofthe appended claims.

What is claimed is:
 1. A near-field electromagnetic probe for convertingan incident energy beam into an interrogating beam which, in anear-field vicinity of the electromagnetic probe, exhibits a transversedimension that is small in relation to the wavelength of the incidentenergy beam, the probe comprising:an energy source for directing saidincident energy beam along a path, said incident energy beam having awavelength λ; an antenna positioned in said path, said antennacomprising a first conductive region and a second conductive regionseparated by a gap, said gap having a transverse dimension that issubstantially less than λ, said antenna, when irradiated by saidincident beam, creating a field across said gap which causes are-radiation of an interrogating beam exhibiting an approximatetransverse dimension of said gap in a near-field region with respectthereto; and a target support for placing a target sufficiently close tosaid gap to be within said near-field region of said interrogating beam.2. The near-field electromagnetic probe as recited in claim 1, furthercomprising;impedance means connected between said first conductiveregion and second conductive region which, in combination withcapacitance across said gap, creates a resonant circuit at a frequencyof said incident energy beam.
 3. The near-field electromagnetic probe asrecited in claim 1, wherein said first conductive region and secondconductive region comprise a bowtie antenna structure.
 4. The near-fieldelectromagnetic probe as recited in claim 1, wherein said firstconductive region and second conductive region are mounted on asubstrate that is substantially transparent to said incident energy beamand said interrogating beam.
 5. The near-field electromagnetic probe asrecited in claim 4, wherein said substrate is continuous in a vicinityof said gap.
 6. The near-field electromagnetic probe as recited in claim1, wherein said incident energy beam exhibits a wavelength λ in avisible range of the electromagnetic spectrum.
 7. The near-fieldelectromagnetic probe as recited in claim 1, wherein said incidentenergy beam is polarized.
 8. The near-field electromagnetic probe asrecited in claim 7, wherein a direction of polarization of said incidentbeam is aligned with a preferred direction of polarization of saidantenna.
 9. The near-field electromagnetic probe as recited in claim 1,wherein said incident energy beam is focused onto said antenna.
 10. Thenear-field electromagnetic probe as recited in claim 9, wherein saidincident energy beam produces a diffraction limited spot with a diameterno greater than 2λ.
 11. The near-field electromagnetic probe as recitedin claim 1, further comprising:means positioned so as to collectradiated energy from said antenna, upon said first conductive region andsecond conductive region being induced to radiate energy as a result ofenergy emitted from said target into said gap.
 12. A near-fieldelectromagnetic probe for use with electromagnetic energy having awavelength λ, said probe comprising:a target support for holding atarget; an antenna comprising at least a first conductive region and asecond conductive region, said first conductive region and secondconductive region separated by a gap, said gap having a transversedimension that is substantially less than λ, said antenna positionedsufficiently close to said target to place said target within anear-field region of said gap; and means for causing electromagneticemission of wavelength λ from said target, with said antenna responsiveto electromagnetic energy of wavelength λ emitted from said target thatis within said gap, to induce currents in said first conductive regionand second conductive region indicative of intensity of said emittedenergy within said gap.
 13. The near-field electromagnetic probe asrecited in claim 12, further comprising:means positioned to collectradiated energy of wavelength λ from said antenna, upon said firstconductive region and second conductive region being induced to radiateenergy as a result of currents induced thereinto by said emittedelectromagnetic energy from said target.
 14. The near-fieldelectromagnetic probe as recited in claim 12, furthercomprising;impedance means connected between said first conductiveregion and second conductive region which, in combination withcapacitance across said gap, creates a resonant circuit at a frequencyof said electromagnetic energy of wavelength λ.
 15. The near-fieldelectromagnetic probe as recited in claim 12, wherein said firstconductive region and second conductive region comprise a bowtie antennastructure.
 16. The near-field electromagnetic probe as recited in claim12, wherein said first conductive region and second conductive regionare mounted on a substrate that is substantially transparent to saidelectromagnetic energy of wavelength λ.
 17. The near-fieldelectromagnetic probe as recited in claim 16, wherein said substrate iscontinuous in a vicinity of said gap.
 18. The near-field electromagneticprobe as recited in claim 12, wherein said wavelength λ is in a visiblerange of the electromagnetic spectrum.
 19. The near-fieldelectromagnetic probe as recited in claim 12, wherein saidelectromagnetic energy of wavelength λ is polarized.
 20. The near-fieldelectromagnetic probe as recited in claim 13, wherein saidelectromagnetic energy of wavelength λ is polarized.
 21. The near-fieldelectromagnetic probe as recited in claim 20, wherein a direction ofpolarization of said electromagnetic energy is aligned with a preferreddirection of polarization of said antenna.